1. Field of the Invention
This invention relates to a method for the production of a bonded wafer comprising two integrally joined wafers.
2. Description of the Prior Art
As a means for bonding semiconductor wafers, the method of bonding a single crystal silicone wafer superposed on a single crystal silicon wafer through the medium of an organic or inorganic adhesive agent has been available to date. This method entails the disadvantage that the part of adhesive bonding in the produced bonded wafer, on thermal expansion, develops stress and manifests strain.
For the solution of this problem, Japanese Patent Publication SHO 62(1987)-27,040 discloses a method which effects bonding of two single crystal silicon wafers by subjecting the surfaces thereof intended for contact to a treatment for impartation of hydrophilic property, then putting them into close mutual attachment, and treating them at an elevated temperature. By this method, the bonding is attained without use of an adhesive agent.
Japanese Patent Application Disclosure SHO 62(1987)-283,655 discloses a method which effects bonding of two semiconductor wafers after causing the surfaces thereof intended for contact to be etched in a vacuum or in an atmosphere of inert gas.
More recently, in view of the prevalent trend of semiconductor devices toward three-dimensional configuration, high voltage breakdown, and dielectric separation between adjacent component elements, wafers of the silicon on insulator (SOI) configuration have come to attract special attention. This configuration comprises two single crystal silicon wafers and an silicon oxide film interposed as an insulating film therebetween.
As a means for the production of a wafer of the SOI configuration, Japanese Patent Publication SHO 62(1987)-34,716 discloses a method for producing an integral thin film of single crystal silicon by forming a thermal oxide film on the surface of a single crystal silicon wafer, forming along the peripheral part of the single crystal silicon wafer a single crystal projected part integrally continuing therefrom, covering the thermal oxide film with a polycrystalline or amorphous silicon film, and causing an energy beam such as an electron beam or a laser beam to irradiate the silicon film linearly and unidirectionally thereby linearly melting the silicon film, and cooling and solidifying the molten silicon film.
This method, however, is found such that though the conversion of the silicon film into a single crystal is partially attained by the interaction of the molten silicon with the oxide film, it is difficult to obtain a single crystal silicon film which privents the method from actual use.
As a means of overcoming this problem and allowing production of a wafer of the SOI configuration by bonding, a method which produces a bonded wafer by directly bonding two single crystal silicon wafers having an oxide film formed on the surface thereof and then reducing one of the silicon wafers into a thin film by etching away the surface region thereof has been disclosed [Tadayoshi Enomoto: "Nikkei Microdevices," No. 15, page 39 (Sep., 1986); and Lasky, Stiffer, White, and Abernathy: "Digest of the IEEE Int. Elec. Device Meeting (IEDM)," page 688 (Dec. 1985)]. To be specific, a high-concentration n.sup.+ silicon substrate having a low-concentration n.sup.- epitaxial layer formed on the surface thereof and a low-concentration supporting silicon substrate for supporting are prepared and thermal oxide films are formed one each on the opposed surfaces of the two substrates. Then, the coated substrates are mutually superposed, pressed closely against each other, and heat-treated in an oxidizing atmosphere at 700.degree. C. thereby to complete bonding of the SiO.sub.2 layers. The variation of the silicon oxide film in thickness from the level of natural oxide film to 520 nm has been demonstrated experimentally Incidentally, the mechanism of the bonding is explained as follows. First, when the O.sub.2 gas present between the wafers is converted into SiO.sub.2, a vacuum part is partially produced and the wafers are forced to stick fast at the part to each other. Once this fast stick is produced, the bonding between the wafers is eventually completed followed by the reactions of hydrogen bonding and dehydration condensation. Then, the SOI configuration is completed by removing the n.sup.+ silicon substrate through selective etching while allowing the n.sup.- epitaxial layer to remain intact.
The method disclosed in Japanese Patent Publication SHO 62(1987)-27,040, however, has the disadvantage that since the wet process using a liquid treating agent is adopted for the treatment of the opposed surfaces for bonding for the impartation of hydrophilic property, the joined surfaces not only introduce a hydrophilic atomic group but also suffer persistence of various kinds of impurities and these defiling substrances, on exposure to a heat treatment, form faults, exert adverse effects on electric properties, and obstruct thorough purification which is indispensable to high integration of any semiconductor device. Further, the strength of bonding is not sufficient owing to the presence of such defiling substances.
The method disclosed in Japanese Patent Application Disclosure SHO 62(1987)-183,655 is capable of solving the drawback of the invention of Japanese Patent Publication SHO 62(1987)-27,040 because it basically effects the etching of the opposed surfaces before their mutual attachment by a dry process. This method, however, has the disadvantage that since this etching process necessitates an expensive and voluminous apparatus and entails a time-consuming work of setting conditions as for the creation of a vacuum prior to the actual etching work, the cost of production is very high and the method is commercially feasibilized only with difficulty.
Further, the method which produces the wafer of the SOI configuration by the bonding described above suffers from the same problem as the method of Japanese Patent Publication SHO 62(1987)-27,040 because it pays no special attention to the presence of defiling substances in the opposed surface prior to bonding. Moreover, the operation of etch back Which is performed in decreasing the wall thickness of one of the two joined wafers necessitates an expensive and voluminous apparatus and jeopardizes the commercial feasibility of the method because the time, ion source, power source, etc. which are required for the etch-back operation invariably contribute to an addition to the cost of production.